Pull dictionary of docinfo fields from a document?

My intention is to use DocInfo as a way to scape meta information off RST
files to build an index of them.

There is a DocInfo transformer at docinfo.transforms.frontmatter.DocInfo.

Two things:

1. I don’t want DocInfo fields to show on HTMLWriter
2. I want to pull a python dictionary of key->value fields from DocInfo.

e.g.

==============
Document title
==============

:date: 2017-05-01
:custom_field: value

rest of the file

Being able to extract:

{
    "date": datetime.date(2017,5,1),
    "custom_field": "value"
}


My understanding is DocInfo handles that fields in biblio_nodes, but also
can handle arbitrary field names. (
http://docutils.sourceforge.net/docs/ref/doctree.html#docinfo). Is that
true?

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